Abstract:
The CELIAS instrument is designed to study the composition of the solar wind (SW) and of solar and interplanetary energetic particles on SOHO. SOHO is a project of international cooperation between ESA and NASA. The CELIAS instrument consists of three different sensors with associated electronics, which are optimized each for a particular aspect of ion composition. These aspects are the elemental, ... isotopic, and ionic charge composition of SW or energetic ions emanating from the Sun. In addition, the Solar EUV Monitor (SEM) has been included into CELIAS for monitoring the absolute EUV flux from the Sun.

Particle abundance observations of the solar wind and of solar energetic particles (SEP) by itself, and in close correlation with optically observable phenomena on the Sun, will allow to tackle basically unsolved questions in solar physics such as: (1) the steady heating process in the corona, (2) SW acceleration processes, (3) dynamic heating phenomena driven by magnetic fields or waves, e.g. heating in coronal mass ejections, filament eruptions, flares, (4) SEP acceleration processes, (5) processes leading to variations in abundances (elemental, ionic charge, isotopic) and (6) the relation between composition and events/regions in the solar atmosphere.

Further scientific goals of CELIAS are to study the composition and dynamics of interplanetary pick-up ions in correlation with the solar EUV flux. For this purpose a solar EUV flux monitor (SEM) for the wavelength 17 to 70 nm has been included into the CELIAS instrument as a sub-unit of the STOF sensor. The SEM serves also to monitor the total absolute EUV flux for the large EUV telescopes on SOHO.

Evidently, the understanding of the phenomena outlined above requires collaboration with many other astrophysical disciplines. Among them are ground based observations including radio astronomical data (e.g. the well known Type III kilometric radio bursts which are related to the strong enrichment of certain species in SEP's) or the interpretation of solar wind abundances in terms of solar abundances as derived from photospheric spectra, meteoritic composition, and from solar oscillation data.